1. Field of the Invention
The present invention relates to an electronic endoscope having an image pick-up unit for picking up images in a body cavity incorporated in the distal end thereof.
2. Description of the Related Art
In the related art, medical diagnosis utilizing an electronic endoscope has widely been carried out in a medical field. The distal end, inserted into a body cavity, of the endoscope scope is internally provided with an image pick-up unit having a solid-state image pick-up device such as a CCD, pick-up signals acquired by the CCD are subjected to signal processing by a processor unit, whereby it becomes possible to observe an image (an endoscopic image) in a body cavity via a monitor display.
A pick-up unit incorporated in the electronic endoscope is structured so that it includes a CCD described above and an object optical system for picking up image light at a portion to be observed in a body cavity, which is made incident through an observation window secured at the distal end of the insertion portion, cover glass is disposed on the image pick-up surface of the CCD with an air gap secured, and a prism is connected to the cover glass and the object optical system.
However, the temperature of the distal end of the insertion portion of an endoscope inserted into a body cavity becomes equivalent to the body temperature (to 37° C.). To the contrary, the temperature inside the insertion portion sometimes becomes 40° C. or more, which is higher than the body temperature, due to driven heat of the electronic components such as a CCD. In addition, since rinse water or air may be jetted onto the distal end of the insertion portion where the observation window is stained, a difference in temperature occurs between the surface of the distal end of the insertion portion and the interior thereof. Accordingly, if moisture is contained in the insertion portion, it has been found that condensation may occur on the object optical system and the cover glass.
In particular, the inside surface of the cover glass is liable to become higher in temperature because it is close to the image pick-up surface of the CCD. On the other hand, since the outer surface of the cover glass to which a prism is connected may be rapidly cooled down by jetting of rinse water, condensation may occur on the inner side of the cover glass due to moisture contained in the air gap.
Also, if the endoscope is connected to a processor unit and its power source is turned on when using an electronic endoscope that was being stored, the temperature of the solid-state image pick-up device rises immediately thereafter. However, the temperature of members such as the object optical system, prism, and cover glass gradually rises while obtaining heat of the solid-state image pick-up device and its peripheral circuits. Therefore, since there is a great difference in temperature between the solid-state image pick-up device and the cover glass immediately after the power source is turned on, condensation is apt to occur.
Where condensation occurs in the object optical system, the image is merely blurred, wherein there is no remarkable influence on observation. However, if condensation occurs on the inside surface of the cover glass as described above, water drops become so significant that they can be viewed on an image, and the image quality deteriorates, wherein it becomes difficult to observe.
In order to prevent condensation as described above, such a solid-state image pick-up unit has been proposed, which is devised so that no moisture is permitted to enter the air gap by enclosing the air gap with a material such as ceramic that does not permeate any moisture (Refer to JP-A-2003-282847). Further, such an image pick-up unit for an endoscope has been proposed (Refer to JP-A-2003-284686), in which a heating body such as peripheral circuits of a solid-state image pick-up device is disposed in the vicinity of the cover glass so as to heat the outer surface (the surface to which the prism is connected) of the cover glass.
However, as in the method described in JP-A-2003-282847, even if such a structure is employed which keeps the air gap airtight, and the cover glass is attached with the air gap kept airtight when manufacturing an image pick-up unit, it is not easy to prevent moisture from intruding. Ultimately, water is permitted to intrude due to chronological changes. Also, since the unit is large-scaled by addition of the structure of keeping the air gap airtight, there is only a slight effect although the production cost is increased.
Further, in the method described in JP-A-2003-284686, since the vicinity of the cover glass is merely heated by a heating body, there is a problem that the utilization efficiency of heat is very poor. Therefore, the temperature is raised not only on the outer surface of the cover glass, but also for the entirety of the image pick-up unit. In addition, there is another problem that it takes much time until the outer surface of the cover glass is heated and the method cannot cope with a radical change in temperature.